1. Field of the Invention
This invention relates to an image forming apparatus, such as a thermal transfer printing apparatus or an electrophotographic copying machine, for forming an image on a recording medium, such as a paper sheet.
2. Description of the Related Art
U.S. Pat. No. 4,975,715 discloses a thermal transfer printing apparatus. In the thermal transfer printing apparatus, a recording medium, for example, a paper sheet or an over head projector (OHP) sheet, is conveyed to a platen drum and is wound around an outer circumferential surface of the platen drum. A thermal head presses an ink ribbon including, for example, four color recording regions (yellow, cyan, magenta and black) against the recording medium wound around the platen drum. The thermal head is then energized and heated such that ink in one or more of the four color regions is melted and transferred onto the recording medium. Thereby, a desired color image may be printed on the recording medium.
The printing apparatus includes a supply sensor for detecting a supply of the recording medium, a type sensor for detecting the type of the recording medium and a size sensor for detecting the size of the recording medium. Thereby, the printing apparatus can form images in accordance with the type and the size of the recording medium detected by the type sensor and the size sensor, respectively, when the supply sensor detects the supply of the recording medium.
However, these sensors may have varying output levels due to the mounting states thereof, the sensitivities thereof and the foreign particles, such as dust, toner or ink within the apparatus. The printing apparatus therefore has to adjust the output levels from these sensors so as to correctly detect the supply of the recording medium, the type of the recording medium and the size of the recording medium.
The conventional apparatus includes an adjusting circuit for adjusting the output levels, such that the output levels may be adjusted by controlling the sensitivity of the adjustment circuit by servicemen. Thus, in the conventional apparatus, it takes a long time to adjust the output levels from the supply sensor, the type sensor and the size sensor. Also, users may not adjust the output levels from the supply sensor, the type sensor and the size sensor.